Titanium-aluminium alloys and inter-metallic compounds (generically termed herein “titanium-aluminium compounds”) are very valuable materials. However, they are difficult and expensive to prepare, particularly in the preferred powder form. This expense of preparation limits wide use of these materials, even though they have highly desirable properties for use in automotive, aerospace and other industries.
Titanium minerals are found in nature in the form of a very stable oxide (TiO2). Common processes for the production of titanium are the Kroll process and the Hunter process. The Kroll process requires the use of magnesium as a reducing agent to reduce TiCl4 (prepared from the oxide by a pre-process of chlorination) to produce the Ti metal. The Hunter process requires the use of sodium as the reducing agent. Because TiCl4 is still thermodynamically stable, highly reactive reducing agents such as magnesium or sodium are required to produce titanium metal out of TiCl4. Such highly reactive reducing agents are difficult and expensive to handle. As the magnesium chlorides in the case of the Kroll process are stable up to temperatures in excess of 1300K, the product is often in the form of a Ti sponge mixed with MgCl2 and remnants of Mg and TiCl2. To obtain pure Ti, the product requires extensive post-processing, including washing and melting in a vacuum arc furnace to remove all impurities. This contributes to the present high cost of the production of titanium.
In the known technologies for production of titanium alloys such as Ti—Al—V, and intermetallic compounds such as Ti3Al, TiAl, TiAl3, Ti—Al—(Cr, Nb, Mo, etc) and alloys based on these compounds, appropriate amounts of sponges, ingots or powders of the metals which comprise these alloys are milled or melted together and annealed, hence adding to the production cost, particularly as it is necessary to obtain the metals first which, as discussed, in the case of titanium, involves considerable expense. For production of a powder of these titanium alloys and intermetallic compounds, further processing is usually required, adding to the already high production cost.
Prior Al-based processes for manufacturing of Ti—Al compounds include starting from Al powder and Ti powder (references: (I. Lu, M. O. Lai and F. H. Froes, Journal of Metals, February 2002, p 62) and (N. Bertolino et al., Intermetallics, Vol 11, 2003, p 41) and reduction of TiCl4 with AlCl (US patent application US2002/0184971 A1). For the first process, the starting materials are Al and Ti powders, the powders usually being mechanically milled to make a uniform mixture followed by heating in a furnace. The resulting materials are at best in the form of solid lumps and this process is usually unable to produce fine powder. Furthermore, the resulting compounds often require heat treatment to produce the required material properties. For the second process, Al metal is heated in the presence of chlorine at temperature around 1200C to produce gaseous AlCl that is then reacted with TiCl4 in the gas phase to produce powders of titanium aluminides. Both these processes are quite complex and costly to operate.
It is also known to perform direct reduction of TiCl4 with aluminium. However, this results in the production of an uncontrollable composition of compounds and production of a single phase material such as TiAl has not been achieved (see in particular S J Gerdemann & D E Alman, page 3341 in Gamma Titanium Alumini 1999, edited by Kim, Dimiduk & Loretto, The Minerals Metals and Materials Society USA).
Over the past several decades, there have been extensive attempts made to replace the existing Kroll and Hunter technologies using techniques such as electrowinning, plasma-hydrogen and also aluminothermic reduction.
The use of hydrogen plasma for the reduction of titanium chloride in a plasma atmosphere is difficult due to unfavourable thermodynamic characteristics, since chlorine preferably reacts with titanium in the reverse reaction to produce titanium chlorides, hence degrading the quality of the produced Ti powder and limiting the efficiency of the method. In a process disclosed in U.S. Pat. No. 5,935,293, a fast quench reactor was used to cool down the plasma in order to prevent recombination processes leading to formation of titanium chlorides. According to the description in U.S. Pat. No. 5,935,293, the process is highly energy expensive relative to the existing Kroll technology.
In another process (G. Z. Chen, D. J. Fray and T. W. Farthing, Nature, Vol 407, (2000), 361), Chen et al. made titanium sponge directly from the oxide by reduction in a molten calcium chloride salt. Oxygen from the titanium oxide recombines with carbon at an anode to form CO2. However, the composition of the resulting sponge-like titanium product produced corresponds to the composition of the starting minerals. The process is still under development and is yet to be demonstrated on an industrial scale.
Attempts have been made to use aluminium as a reducing agent for TiCl4 in plasma systems. For reduction of TiCl4 using aluminium, the products are in the form of solid phase titanium-aluminium intermetallic compounds mixed with aluminium chloride and some residual titanium dichloride. A description of various attempts using aluminium together with a description of the thermodynamics of the process are given by Murphy and Bing (High Temp. Chem. Processes, Vol 3, 365-374, 1994). Because of difficulties associated with gas phase reactions it has not been possible to produce titanium and/or titanium-aluminium compounds by direct aluminothermic reduction of titanium chlorides.